Large skeletal defects frequently arise following reconstructive surgery for trauma, cancer resection, infection and congenital deformity. The standard clinical treatment for these defects is the transplantation of a bone graft derived from the patient (autograft) into the defect site. Although an autograft, typically, does not cause an immune response, it is associated with several disadvantages including donor site morbidity and limited availability. Furthermore, it is possible that an autograft will not successfully vascularize in the new site, leading to its resorption and/or infection of the graft site.
Other treatments may be used to replace the defect. These include implantation of a bone cement (calcium phosphate, acrylic [e.g., PMMA], or plastic [e.g., polyethylene]) or a metal prosthetic. Unfortunately, the long term persistence of these materials can lead to complications at and surrounding the defect site. For example, because such materials may have a different modulus and strength than the surrounding tissue, implantation of prosthetic devices made from such materials into the defect site can lead to damage of the surrounding tissues. Moreover, prosthetic devices made from these materials do not (a) revascularize or allow for tissue regeneration in the defect site, (b) integrate with the existing host bone, or (c) undergo subsequent remodeling in response to evolving mechanical (i.e., ambulatory loading or protection of internal organs) needs. This leads to problems with protective strength and aesthetics as the host bone remodels around the implanted prosthetic. The problems are magnified by the post-surgical need to protect the brain or other internal organs covered by prosthetic from infection and trauma during the healing process.
The inadequacies of the current prosthetics have led to an investigation of prosthetics that act as porous scaffold for attachment and proliferation of tissue progenitor (e.g., bone progenitor) cells. It has been proposed that such scaffolds be made from a biodegradeable polymer that slowly degrades away, eventually leaving a defect site that has been repaired by the cells which migrate into and attach to the scaffold. Prior to its degradation, the scaffold serves not only to provide a surface for attachment of the progenitor bone cells, but also to allow in nutrients and to support any biomechanical functions (ambulatory loading or protection).
Polymeric scaffolds have been fabricated by a chemical cross-linking process or, more recently, by photocrosslinking the liquid polymeric material in a transparent mold. Molds do not facilitate the formation of internal spaces such as pores. To overcome this deficiency, investigators have tried to stir salt crystals into the polymer and later leach it out with water. The salt crystal space is left as a pore, but the surface texture and arrangement of pores is not homogenous and geometrically may not be conducive to cell attachment, proliferation, or maturation, as well as vascular ingrowth.
Accordingly, it is desirable to have new methods for fabricating biodegradable polymeric prosthetics for treating critical size bone defects. It is also desirable to have a method which provides a prosthetic that not only maintains its shape and protective strength during the healing process but also encourages bone ingrowth and incorporation of the prosthetic with the host bone. It is also desirable to have a method for fabricating a prosthetic which comprises a polymer that, when prosthetic implanted into a defect site, does not degrade into by-products that have serious adverse affects on the cells and tissues in contact with the prosthetic.